I posted this in another thread, but I'll post it here too:
Look at it from Ron Paul's point of view. He doesn't even like running for President. He said he tried a third party before, and didn't like it then. Nothing in life is guaranteed, and something drastic could happen that would make him want to be a 3rd party candidate, but it's very unlikely anything that drastic would happen. That's why he always says he's 99.99% sure he's not running for a 3rd party. Nothing is guaranteed in life, but he's not planning on anything past the GOP primary. And I believe him.
Put yourself in his shoes. He's probably tired of campaigning. He likes the attention he got, and is amazed at all his new supporters. But at the end of the day, he's a very reluctant politician, and doesn't want to be flying around the country for the next 10 months, running for President. He said it's hard to get on ballots, and since you can't get into the debates, what's the point? He knows he'll just be ignored by the media, and figures in the end, he'll get just enough votes to beat the margin of error.
He doesn't "owe" his supporters a 3rd party run. He's never hinted that he would run 3rd party if his GOP bid fails. He's been in dozens of nationally televised debates, and gotten his message out to a large crowd. Most people who don't care about politics, won't care about his views. Those that do care, already know about his platform and ideas. So yeah, I suspect he is being honest every time he says "I have no intention of running". We trust Ron Paul on everything else, let's trust him on that.